To improve connection rigidity and to reduce the number of parts by integrally forming an attaching surfaced to which a supporting device for supporting the weight load of a power unit and an acceleration support part for supporting an acceleration shaft of a final reduction gear housed in a differential gear case so as to be rotated.

The rear part of a lower case 4 as one of the upper part constituting parts of an oil pan attached to an engine 1 is extended backward up to a joined connection part between the engine and a transmission case 2 and the lower case 4 is caused to have a connection function between the engine 1 and the transmission case 2, a bracket function for attaching of a supporting device 7 and a function for supporting an acceleration shaft so as to be rotated. Thus, such conventionally needed parts as a stiffener, a mount bracket and an acceleration support are made unnecessary, the number of parts is reduced, the peripheral structure of an engine rear part is simplified and costs are greatly reduced.